It has been recognized for a number of years that interruptions in breathing activity of infants, and particularly of premature infants, are significant medical events. Such interruptions of breathing are known as apnea. In addition to interruptions of breathing effort, another type of apnea involves a physical blockage or obstruction of the infant airway. This form of apnea is known as obstructive apnea and is not associated with a reduction in respiratory effort of the infant.
Although apnea by itself is a significant medical event, it is also recognized that when apnea is coupled with bradycardia (a significant slowing in heart rate unique to newborns), far more serious medical consequences may result. Various medical studies have proposed historical reconstruction of correlations between apneic events and bradycardic events for the purpose of diagnosis. Such reconstructions are typically performed using chart recording instruments having a common time scale and allowing these instruments to record long durations of patient information. Typically, many yards of chart paper are collected over a twelve-hour or longer duration and are aligned for visual inspection by an attending physician or researcher.
The instruments used in these correlation studies have included impedance pneumographs and conventional heart rate monitors.
Impedance pneumography is based on the principle that volume changes within a conducted electrical field are accompanied by changes in electrical resistance. Typical impedance pneumographs impose a small current (less than 0.3 ma) at 40 to 100 kiloHertz through electrodes placed on the infant chest wall.
Instruments which combine respiratory and cardiac information and plot the derived data on paper, display it on screen, or record it to a magnetic tape have been available for some time. In addition, many of these instruments are equipped with alarm circuitry which provides a visual or audible indication to attending medical personnel. These alarms are typically adjustable over some defined range. For example, periods of decreased breathing activity exceeding 20 seconds are determined to be apneic events and an apnea alarm is sounded.
A non-invasive technique for measuring oxygen saturation in active hemoglobin may be provided by a pulse oximeter (Nonin Medical Model 13030) which uses both visual and infra-red wavelengths of light transmission through, for example, a fingertip (or in the case of an infant, the big toe). Percent oxygen saturation of oxyhemoglobin is important as an indicator of the physiological condition which results from episodes of apnea, or episodes of apnea coupled with bradycardia.